Thermal stability of trifluoroiodomethane (R13I1) as an environmentally friendly working fluid for refrigeration and heat pump systems: Investigations and improvement strategies

Abstract

Trifluoroiodomethane (R13I1) has attracted extensive attention because of its excellent environmental protection performance and flame retardant effect. However, R13I1 has poor thermal stability due to the presence of C-I bonds, which greatly limits its application in refrigeration and heat pump systems. At present, there are few theoretical and experimental studies on the thermal stability of R13I1 and its improvement methods. In order to solve these problems, the thermal stability and pyrolysis mechanism of R13I1 and its mixture with lubricants and stabilizers were investigated using experiments, DFT calculations, and ReaxFF simulations in this study. Firstly, the pyrolysis temperature and pyrolysis products of R13I1 were measured using the reactor method. The initial pyrolysis temperature range of R13I1 was 110–120 °C. C2F6 and I2 are the main pyrolysis products of R13I1. Then, the effect of polyol ester (POE) oil on the thermal stability of R13I1 was analyzed. The results showed that the addition of POE oil significantly promoted the decomposition of R13I1 and the generation of the pyrolysis product CHF3. Circulation systems containing R13I1/POE oil were not suitable for long-term operation at temperatures above 120 °C. The active radicals generated by the pyrolysis of lubricant were the key factor in the deterioration of the thermal stability of R13I1. Finally, a stabilizer addition to the lubricant was used to improve the thermal stability of the R13I1/POE oil, and the optimal stabilizer was determined to be 5 % AN5/0.15 % 2-ethylhexyl glycidyl ether. The thermal stability of R13I1/POE oil before and after the addition of the optimal stabilizer was compared. The experimental results showed that when the reaction temperature was 160 °C, the concentration of R13I1 increased from 98.94 % to 99.85 %, the concentration of CHF3 decreased from 0.89 % to 0.08 %, and the pyrolysis rate of R13I1 was only 3.86 × 10−9 s−1 with the addition of the optimal stabilizer. The optimal stabilizer achieved excellent improvements. This study not only provides basic thermal stability data for R13I1 and its mixtures, but also proposes effective methods to improve the thermal stability, which is of great significance for the safe and reliable application of R13I1 in refrigeration and heat pump systems.